Scale assembly

ABSTRACT

A guideway 10 of a guideway set in a coordinate measuring machine has a scale in the form of a metal strip clamped and bonded to an accessible face 46 of a spar 30, the latter being formed of a thermally stable material and being connected at one end to a reference position defined by a bracket 18 and a mounting plate 16 on the guideway 10. The spar 30 is mounted on the guideway 10 by spaced blocks 34 which are designed to accommodate any longitudinal movement of the guideway 10 due to thermal expansion, so that the spar 30 and the scale are effectively maintained in position. Because relative movement between the spar 30 and the scale is substantially prevented, the position of the scale is substantially fixed, providing improved scale accuracy.

This invention relates to a scale assembly for use in monitoringmovement of a component on a structure, and is particularly but notexclusively concerned with monitoring movement of a component on alinear guideway of a high accuracy measuring and checking machine.

For purposes of accuracy, it is necessary to construct coordinatemeasuring machines using either heavyweight materials, such as granite,which remain as stable as possible with variations in temperature, inorder that guideways of the machine do not distort and thereby detractfrom the accuracy of measurement, or special designs of mountings forthe guideways which can accommodate differential thermal expansionbetween the materials used. However, irrespective of the constructionused, there remains a problem with scales which are mounted on theguideways for use in a non-contact linear encoder system. Some suchscales have been made of a non-thermally stable material and, as theguideways require to be made of a material whose surface can be machinedto a high accuracy, and therefore inevitably a non-thermally stablematerial, there still results a differential thermal expansion betweenthe materials off the scale and the guideway. Other scales are expensiveand have length limitations.

In DE-U-9205393, an arrangement for measuring the relative positions oftwo objects mounts a scanning unit on one of the objects, positioned toscan a graduated scale on the other object in order to obtain positionaldata. The design of the prior device attempts to make the scalepredictable in terms of length change due to temperature change, inorder to allow the user to be able to compensate for that change. Theintention is to avoid the need for all the various components to be madefrom materials with the same coefficient of thermal expansion. Such adesign is not, however, appropriate for use in a high accuracy measuringand checking machine, where it is desirable to maintain the scale in thesame spatial position regardless of temperature changes.

According to the present invention there is provided a scale assemblyfor use in monitoring movement of a component on a guideway structure,the assembly comprising an elongate scale, a support for the scale, andmeans for fixedly mounting the support on the structure at one referencelocation and flexibly mounting the support on the structure at anotherlocation, characterised in that, in order to substantially prevent achange in the spatial position of the scale due thermal expansion ofmaterials used, the support is formed of a thermally stable material,and means fixedly secures the scale to the support to substantiallyprevent relative movement therebetween, the positions of the fixed andflexible mountings for the support on the structure accommodatingmovement of the latter due to thermal expansion thereof, withoutchanging the spatial position of the support and therefore the scale.

Preferably the scale is in the form of a metal tape which may beproduced to have an indefinite longitudinal dimension and may be cut toa required length. The securing means may comprise an adhesive layerprovided on a face of the tape opposed to a scaled face thereof, and acomponent for clamping each end of the tape to the support.

Preferably also the support is formed as an elongate strip, for exampleof unidirectional carbon fibre material. Alternatively the strip may beformed of INVAR or the like. The strip may be formed with an indefinitelongitudinal dimension and may be cut to a desired length.

The flexible mounting means for the strip may include a plurality ofblocks, each of which may be formed of two hinged parts, one of whichparts is secured to the support and the other of which parts adapted tobe secured to the structure. Each block may be formed of aluminum withan integrally formed flexible section defining the hinge.

Alternatively the flexible mounting means for the strip may comprise asealant which can be applied in flowable state to extend between eachside of the support and the structure over part all of the length of theformer, the sealant remaining flexible when set.

A tape matching the scaled tape may be secured to an opposed face of thesupport, and the or each tape may locate in a complementary recess inthe support whereby the or each respective face of the latter remainsplanar.

The present invention also provides apparatus for use in monitoring themovement of a component, said apparatus comprising elongate guide meanson which the component is adapted to be mounted for movementthere-along, characterised by a scale assembly as described in any ofthe six preceding paragraphs mounted in association with the elongateguide means, the component providing reading means for non-contactassociation with the scale for monitoring movement of the component.

The invention Further provides a coordinate measuring machinecharacterised by an apparatus as described in the preceding paragraphfor guiding a respective component along at least one of the coordinateaxes.

An embodiment of the present invention will now be described by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a side view of a guideway set for a coordinate measuringmachine incorporating a scale assembly according to the invention;

FIG. 2 is a view of the guideway set of FIG. 1 in the direction of arrowB;

FIG. 3 is a section along line 3--3 of FIG. 2; and

FIG. 4 is a plan of a component of the scale assembly.

Referring to the drawings, there is shown a guideway 10 of guideway setin a coordinate measuring machine adapted to mount a carriage (notshown) for movement on a column along the Z axis, i.e. the guideway 10is upstanding in use having a lower end 12 and an upper end 14. Theguideway 10 has a tubular form of octagonal cross-section as shown inFIG. 3, and is preferably made of a ceramic material bonded at its lowerend 12 to a mounting plate 16, the mounting position of the latter beingnon-movabie relative to the column of the machine, and therefore beingan effective reference position.

A substantially L-shaped bracket 18 has one leg 20 attached to themounting plate 16 with the other leg 22 providing a mounting face spacedoutwardly of a respective end wall 26 of the guideway 10 for a purposehereinafter described.

The machine is provided with a non-contact linear encoder systemincluding a scale assembly mounted along the end wall 26 of the guideway10. The scale assembly consists of a scale 28 in the form of a metalstrip, for example a gold-plated steel strip which is accurately scaledon one face and on the opposed face has a layer of adhesive. The scale28 is normally provided with a removable cover over the adhesive and inroll form whereby the scale 28 can be cut to any desired length. Theadhesive is preferably a contact adhesive whereby the scale 28 can bebonded to a desired substrate by the application of manual pressure.

The scale assembly further includes an elongate component in the form ofa strip or spar 30 formed of a thermally stable material, preferablyunidirectional carbon fibre. The spar 30 has a rectangular cross-sectionand can be accurately formed and then cut to any desired length. As analternative, the spar 30 may be formed of any other suitable thermallystable material such as INVAR. To one side face 32 of the spar 30 areattached a plurality of blocks 34 for mounting the spar 30 on the endwall 26 of the guideway 10. The blocks 34 are optimally spaced along thelength of the spar 30.

As shown in FIG. 4, each block 34, preferably formed of aluminum, isformed of two parts, a first part 36 being provided with mountingapertures 38 through which suitable fixing screws 40 (FIG. 1) can securethe block 34 to the spar 30. The second part 42 is adapted to be bonded,at the opposed side from the spar 30, to the end wall 26 of the guideway10. The parts 36, 42 are connected at opposite edges by an integrallyformed strip 44 so dimensioned as to enable flexibility and thereforeacting effectively as a hinge between the parts 36, 42.

After the blocks 34 are mounted on the spar 30, but before the blocks 34are bonded to the guideway 10, the spar 30 is secured at one end to thebracket 18. The spar 30 then extends upwardly parallel to the guideway10 and the blocks 34 are secured to the guideway end wall 26.

The scale 28 with the adhesive side-exposed is then applied to theaccessible side face 46 of the spar 30, and each end of the scale 28 isclamped to the spar 30 by a suitable clamping assembly 48, the latterincluding the bonding of a clamping component in position over therespective end of the scale 28. The cross-section of the spar 30 isselected to be substantially larger than the cross-section of the scale28. This is because the coefficient of expansion-of the compositeassembly is dependent upon the coefficient of expansions of the twomaterials and their relative cross-sectional areas.

The connection of the thermally stable spar 30 to a reference positioneffectively maintains the position of the spar 30, and although thelatter is mounted on the guideway 10, any longitudinal movement of thelatter due to thermal expansion can be accommodated by the hinged blocks34. As the mounting of the scale 28 on the spar 30 substantiallyprevents relative movement between these two components, the position ofthe scale 28 is substantially fixed, providing considerably improvedscale accuracy relative to existing scale arrangements.

The mounting of the guideway on a thermally stable part of the machineby means of specially designed bars (not shown), which can accommodatedifferential thermal expansion between different materials, is providedfor by means of the blocks 34, which space the spar 30 from the end wall26 of the guideway 10 so that the spar 30 can overlie protruding ends 50of the bars. Where such bars are not utilized, the flexible mounting ofthe spar 30 on the guideway 10 can alternatively be achieved by means ofa sealant strip along the length of the spar 30 and extending between alower edge thereof and the end wall 26 of the guideway 10. The sealantstrip may be formed of silicon and may alternatively be used in smallsections spaced along the length of the spar 30.

In use, a reading head 52 is mounted for movement with the component andscans the scale 28. A draught shield 54 is provided. The head 52, whichforms no part of the present invention, produces signals forinterpolation by an associated computer, with the fixing of the scale 28providing a suitable reference point to assist in interpolation.

It is to be appreciated that the scale assembly can be applied to anyguideway of a coordinate measuring machine and can be used in othersuitable applications. Depending on the guideway design, it may be moreappropriate to fix the scale assembly to a reference position bysecuring the spar 30 with respect to one of the bars mounting theguideway on the column.

In a modification, the spar 30 may be formed with an elongate groove onthe side face 32 wherein the scale 28 can locate to thereby lie flushwith the remainder of the face 32. It is also envisaged that it may bepreferable to counterbalance any distortion caused by thermal expansionof the metal scale 28, however small, by the application of a similarmetal strip, not necessarily scaled, to the other side face of the spar30. It will also be appreciated that, if such an additional strip isused, the spar 30 may also be formed with a central groove toaccommodate same.

There is thus provided a long axis scale for a coordinate measuringmachine which is considerably less susceptible to distortion due tothermal expansion than presently existing scales, and yet which isinexpensive to produce and simple to install. The accuracy of a machineutilizing such a scale assembly is considerably enhanced.

Various other modifications may be made without departing from theinvention. For example, the flexible blocks may be of any other suitabledesign, and indeed the spar may be flexibly mounted on the guideway byany other suitable arrangement.

I claim:
 1. A scale assembly for use in monitoring movement of acomponent on a guideway structure of a coordinate measuring machine, theassembly comprising an elongate scale, a support for the scale, mountingmeans for fixedly mounting the support relative to the structure at onereference location and flexibly mounting the support on the structure atanother location, securing means for fixedly securing the scale to thesupport to substantially prevent relative movement therebetween, andconnecting means for non-movably connecting the structure to themachine, the support being fixedly mounted on the connecting means andbeing formed of a thermally stable material whereby to be capable offixing the spatial position of the support relative to the machine, theflexible mounting for the support on the structure accommodatingmovement of the latter due to thermal expansion of the material thereof,without changing the spatial position of the support and therefore thescale.
 2. An assembly according to claim 1 wherein the scale is in theform of a metal tape.
 3. An assembly according to claim 2 wherein thetape is produced to have an indefinite longitudinal dimension and is cutto a required length.
 4. An assembly according to claim 2 wherein thesecuring means comprises an adhesive layer provided on a face of thetape opposed to a scaled face thereof, and a component for clamping eachend of the tape to the support.
 5. An assembly according to claim 1wherein the support is formed as an elongate strip having an indefinitelongitudinal dimension and being cut to a desired length.
 6. An assemblyaccording to claim 5 wherein the strip is formed of unidirectionalcarbon fibre material.
 7. An assembly according to claim 5 wherein thestrip is formed of INVAR.
 8. An assembly according to claim 1 whereinthe flexible mounting means includes a plurality of blocks, each ofwhich may be formed of two hinged parts, one of which parts is securedto the support and the other of which parts is adapted to be secured tothe structure.
 9. An assembly according to claim 8 wherein each block isformed of aluminum with an integrally formed flexible section definingthe hinge.
 10. An assembly according to claim 1 wherein the flexiblemounting means comprises a sealant which can be applied in a flowablestate to extend between each side of the support and the structure overpart or all of the length of the former, the sealant remaining flexiblewhen set.
 11. Apparatus for use in monitoring the movement of acomponent, said apparatus comprising elongate guide means on which thecomponent is adapted to be mounted for movement therealong, and a scaleassembly according to claim 1 mounted in association with the elongateguide means, the component providing reading means for non-contactassociation with the scale for monitoring movement of the component. 12.A coordinate measuring machine having an apparatus according to claim 11for guiding a respective component along at least one of the coordinateaxes.